In a case with potentially broad ramifications for electronic commerce, an up-and-coming online bookseller said it will plead guilty to intercepting e-mail messages from Amazon.com , the Seattle-based industry leader, to book dealers.

Alibris (www.alibris.com ), a rare-book dealer based in Emeryville, Calif., captured and stored more than 4,000 e-mail messages in an attempt to collect intelligence on the market, according to court...